Cathode active material, its manufacturing method, cathode, its manufacturing method, and secondary battery

ABSTRACT

A secondary battery having a cathode, an anode, and an electrolyte is provided. The cathode includes a cathode active material containing at least one kind selected from the group consisting of sulfur S and phosphorus P in a portion near the particle surface of a lithium composite oxide. A content of the kind in the portion is larger than that in the particle of the lithium composite oxide.

CROSS REFERENCES TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.11/763,222 filed on Jun. 14, 2007, which claims priority to JapanesePatent Application JP 2006-168090 filed in the Japanese Patent Office onJun. 16, 2006, the entire contents of which are hereby incorporated byreference.

BACKGROUND

The present disclosure relates to a cathode active material, itsmanufacturing method, a cathode, its manufacturing method, and asecondary battery and, more particularly, to a cathode active materialcontaining a lithium composite oxide.

In recent years, in association with the realization of advancedperformance and multi-function of mobile equipment, the realization of alarge capacitance of a secondary battery as a power source of suchmobile equipment is earnestly requested. As a secondary battery whichcan satisfy such a request, a large attention has been paid to anon-aqueous electrolyte secondary battery in which lithium cobalt acidis used as a cathode, graphite is used as an anode, and an organic mixedsolvent containing a lithium salt supporting electrolyte is used as anelectrolyte.

In the non-aqueous electrolyte secondary battery in the related artwhich operates at the maximum voltage of 4.2V, the cathode activematerial such as lithium cobalt acid which is used as a cathode uses acapacitance of up to about 60% of its theoretical capacitance.Therefore, a residual capacitance can be used in principle by furtherraising the charge voltage. Actually, it has been known that a highenergy density can be realized by raising an upper limit voltage uponcharging to 4.25V or more (for example, refer to Patent Document 1:pamphlet of International Publication No. 03/019713). To satisfy arequest for the realization of a larger capacitance, in recent years, ananode of a large capacitance using silicon Si, germanium Ge, tin Sn, orthe like has also vigorously been examined.

The foregoing non-aqueous electrolyte secondary battery is mainly usedin mobile equipment such as notebook-sized personal computers, cellularphones, or the like and is often subjected to a relatively hightemperature environment for a long time due to heat which is generatedfrom the equipment, heat in a moving vehicle, or the like. If thenon-aqueous electrolyte secondary battery in a charging state is left insuch an environment, a gas is generated by a reaction of the cathode andan electrolytic solution. Particularly, in the non-aqueous electrolytesecondary battery whose upper limit voltage upon charging has been setto 4.25V or more, the reaction of the cathode and the electrolyticsolution increases and an amount of generated gas increases.

For example, in the case where the non-aqueous electrolyte secondarybattery has been enclosed in a sheathing member made of a laminate film,if the gas is generated as mentioned above, sometimes, such a problemthat the sheathing member is expanded, its thickness increases, and itssize is out of the standard of a battery enclosing portion of electronicequipment occurs. Such a problem that an internal resistance of thebattery increases by the reaction of the cathode and an electrolyticsolution and it is difficult to take out a sufficient capacitance alsooccurs.

It is, therefore, desirable to provide a cathode active material, itsmanufacturing method, a cathode, its manufacturing method, and asecondary battery in which excellent high-temperature preservingcharacteristics can be obtained.

SUMMARY

According to an embodiment, there is provided a cathode active material,wherein at least one kind selected from the group consisting of sulfur Sand phosphorus P is contained in a portion near a particle surface of alithium composite oxide, and a content of the kind in the portion islarger than that in the particle of the lithium composite oxide.

According to another embodiment, there is provided a secondary batterycomprising a cathode, an anode, and an electrolyte, wherein the cathodecontains a cathode active material containing at least one kind selectedfrom the group consisting of sulfur S and phosphorus P in a portion neara particle surface of a lithium composite oxide, and a content of thekind in the portion is larger than that in the particle of the lithiumcomposite oxide.

According to another embodiment, there is provided a cathode containinga cathode active material containing at least one kind of sulfur S andphosphorus P in a portion near a particle surface of a lithium compositeoxide, wherein a content of the kind in the portion near the particlesurface of the lithium composite oxide is largest.

According to an embodiment, there is provided a secondary batterycomprising a cathode, an anode, and an electrolyte, wherein the cathodecontains a cathode active material containing at least one kind selectedfrom the group consisting of sulfur S and phosphorus P in a portion neara particle surface of a lithium composite oxide, and a content of thekind in the portion is largest in the cathode.

According to another embodiment, there is provided a secondary batterycomprising a cathode, an anode, and an electrolyte, wherein in a surfaceanalysis by a Time-Of-Flight Secondary Ion Mass Spectroscopy (TOF-SIMS),the cathode has peaks of fragments of at least one or more secondaryions selected from positive secondary ions of Li₄PO₄, Li₂CoPO₄,Li₂CoPH₂O₄, Li₃CoPO₄, and Li₃CoPO₄H and negative secondary ions of PO₂,LiP₂O₄, Co₂PO₄, CoP₂O₅, CoP₂O₅H, CoP₂O₆, and CoP₂O₆H.

According to another embodiment, there is provided a manufacturingmethod of a cathode active material, comprising the steps of: preparinga solution by mixing a lithium composite oxide particle, at least onekind selected from the group consisting of a sulfur-contained compoundand a phosphorus-contained compound, and a solvent; and drying thesolution.

According to an embodiment, there is provided a manufacturing method ofa cathode, comprising: preparing a cathode mixture slurry by mixing alithium composite oxide particle, at least one kind selected from thegroup consisting of a sulfur-contained compound and aphosphorus-contained compound, and a solvent; coating a cathodecollector with the cathode mixture slurry; drying the cathode collectorafter preparing a cathode mixture slurry to from a cathode activematerial layer.

According to the embodiment, it is preferable that the lithium compositeoxide has average compositions expressed by Formula 1.Li_(x)Co_(1-y)M_(y)O_(b-a)X_(a)  (Formula 1)

in Formula 1, M denotes an element of one kind selected from boron B,magnesium Mg, aluminum Al, silicon Si, phosphorus P, sulfur S, titaniumTi, chromium Cr, manganese Mn, iron Fe, cobalt Co, nickel Ni, copper Cu,zinc Zn, gallium Ga, yttrium Y, zirconium Zr, molybdenum Mo, silver Ag,tungsten W, indium In, tin Sn, lead Pb, and antimony Sb; X denotes ahalogen element; x indicates a value within a range of 0.2<x≤1.2; yindicates a value within a range of 0≤y≤0.1; b indicates a value withina range of 1.8≤b≤2.2; and a indicates a value within a range of 0≤a≤1.0.

According to the embodiment, it is preferable that the content of thekind in the portion lies within a range from 0.1 at % or more to lessthan 5 at % as a ratio to cobalt Co.

According to the embodiment, it is preferable that sulfur S is containedas Li₂SO₄ in the portion and phosphorus P is contained as Li₃PO₄ orLiCoPO₄ in the portion of the lithium composite oxide.

According to the embodiment, it is preferable that a center particlediameter ranges from 1 μm to 30 μm.

According to the embodiment, it is preferable that a specific surfacearea lies within a range from 0.1 m²/g or more to less than 1 m²/g.

According to the embodiment, it is preferable that the anode contains acarbon material or a metal material which can dope and dedope alkalimetal ions. It is preferable that the carbon material contains at leastone kind selected from a group containing graphite, easy-graphitizablecarbon, and non-easy-graphitizable carbon. It is preferable that themetal material contains at least one kind selected from a groupconsisting of silicon Si, tin Sn, and germanium Ge.

According to the embodiment, it is preferable that the electrolytecontains a compound of fluorinated cyclic or chain-like carbonate inwhich a part or all of hydrogen is fluorinated. It is preferable thatthe compound is difluoro ethylene carbonate.

According to the embodiment, it is preferable that the open circuitvoltage per pair of cathode and anode in a full charging state rangesfrom 4.25V to 4.6V. It is more preferable that the open circuit voltageper pair of cathode and anode in the full charging state lies within arange from 4.35V or more to 4.6V or less.

According to the embodiment, a thin coating film containing at least onekind of phosphorus P and sulfur S is formed on the surface of thelithium composite oxide particle. It is presumed that since an activityof the coating film to the electrolyte is low, the reaction of thecathode and the electrolytic solution in the high-temperature preservingstate can be suppressed and the gas generation and the increase ininternal resistance can be suppressed.

As described above, according to the embodiments, the gas generation andthe increase in internal resistance due to the reaction of the cathodeand the electrolytic solution in the high-temperature preserving statecan be suppressed. Therefore, the secondary battery which is excellentin the high-temperature preserving characteristics can be provided.

Other features will be apparent from the following description taken inconjunction with the accompanying drawings, in which like referencecharacters designate the same or similar parts throughout the figuresthereof.

Additional features and advantages are described herein, and will beapparent from, the following Detailed Description and the figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a perspective view showing an example of a construction of asecondary battery according to the first embodiment;

FIG. 2 is a cross sectional view taken along the line II-II in a batteryelement 10 shown in FIG. 1;

FIG. 3 is a cross sectional view showing an example of a construction ofa secondary battery according to the third embodiment;

FIG. 4 is a cross sectional view enlargedly showing a part of a windedelectrode member 30 shown in FIG. 3;

FIG. 5 is a surface diagram showing a result of a TOF-SIMS surfaceanalysis of a cathode active material in Example 4;

FIG. 6 is a graph showing a result according to a TOF-SIMS positivesecondary ion analysis of a cathode in each of Example 7 and Comparison1;

FIG. 7 is a graph showing a result according to a TOF-SIMS negativesecondary ion analysis of the cathode in each of Example 7 andComparison 1;

FIG. 8 is a cross sectional view showing a result of the TOF-SIMSsurface analysis of the cathode in Example 7; and

FIG. 9 is a cross sectional view showing a result of the TOF-SIMSsurface analysis of the cathode in Comparison 1.

DETAILED DESCRIPTION

(1) First Embodiment

(1-1) Cathode Active Material

In a cathode active material according to the first embodiment, asurface layer is formed in at least a part of a lithium composite oxideparticle serving as a center portion. The lithium composite oxide is,for example, a lithium cobalt composite oxide containing lithium andcobalt. Average compositions of the lithium cobalt composite oxide areshown by, for example, Formula 1.Li_(x)Co_(1-y)M_(y)O_(b-a)X_(a)  (Formula 1)

in Formula, M is an element of one kind selected from boron B, magnesiumMg, aluminum Al, silicon Si, phosphorus P, sulfur S, titanium Ti,chromium Cr, manganese Mn, iron Fe, cobalt Co, nickel Ni, copper Cu,zinc Zn, gallium Ga, yttrium Y, zirconium Zr, molybdenum Mo, silver Ag,tungsten W, indium In, tin Sn, lead Pb, and antimony Sb; X is a halogenelement; x indicates a value within a range of 0.2<x≤1.2; y indicates avalue within a range of 0≤y≤0.1; b indicates a value within a range of1.8≤b≤2.2; and a indicates a value within a range of 0≤a≤1.0.

The surface layer functions as a reaction suppressing layer and containsat least one or more kinds of sulfur S and phosphorus P. A content of atleast one kind of sulfur S and phosphorus P in the surface of thelithium composite oxide particle is larger than that in the lithiumcomposite oxide particle. Those materials are contained, for example, asa compound in the surface layer. More specifically speaking, sulfur S iscontained as, for example, Li₂SO₄ in the surface layer. Phosphorus P iscontained as, for example, Li₃PO₄ or LiCoPO₄ in the surface layer.

It is preferable that the content of the kind in the portion ranges from0.1 at % to 5 at % as a ratio to cobalt Co of the cathode activematerial. This is because if it is set to be equal to 0.1 at % or more,excellent preserving characteristics can be obtained and if it is set tobe smaller than 5 at %, an increase in internal resistance and adecrease in capacitance can be suppressed.

It is preferable that a center particle diameter of the cathode activematerial lies within a range from 1 μm or more to less than 30 μm. Thisis because if it is set to be equal to 1 μm or more, the reaction of thecathode and the electrolytic solution is suppressed and an increase ingas generation amount can be suppressed, and if it is set to be lessthan 30 μm, the sufficient capacitance and excellent loadcharacteristics can be obtained.

It is preferable that a specific surface area lies within a range from0.1 m²/g or more to less than 1 m²/g. This is because if it is set to beequal to 0.1 m²/g or more, the sufficient capacitance and the excellentload characteristics can be obtained and, if it is set to be less than 1m²/g, the reaction of the cathode and the electrolytic solution issuppressed and the increase in gas generation amount can be suppressed.

The cathode active material is obtained by, for example, inserting thelithium composite oxide particle into an aqueous solution containing atleast one kind of a sulfur-contained compound and a phosphorus-containedcompound and kneading it, and drying it after that. As a drying method,for example, a hot air type fixed shelf drier, a spray dry, or the likecan be used. A surface product can be also stabilized by thermallyprocessing an obtained dried substance. As a sulfur-contained compound,one, two, or more kinds of sulfur-contained compounds can be used. As asulfur-contained compound, for example, sulfuric acid, sulfurous acid,ammonium sulfate, ammonium hydrogensulfate, organic sulfate, or the likecan be mentioned. As a phosphorus-contained compound, one, two, or morekinds of phosphorus-contained compounds can be used. As aphosphorus-contained compound, phosphoric acid, phosphorous acid,hypophosphorous acid, ammonium phosphate, ammonium hydrogenphosphate,organic phosphate, or the like can be mentioned.

As a method of confirming that at least one kind of sulfur S andphosphorus P exists in the surface, in a surface analysis of an SEM-EDS(Scanning Electron Microscopy-Energy Dispersion X-ray Spectrometry), amethod of confirming by comparing an atom ratio of at least one kind ofsulfur S and phosphorus P existing in the surface to cobalt Co with aratio of materials used for manufacturing the cathode active materialcan be mentioned. The above confirmation can be also similarly made byusing an XPS (X-ray Photoelectron Spectroscopy). After the cathodeactive material was embedded into the resin and its cross sectionalsurface was exposed, distribution in the cross sectional surface can beconfirmed by the TOF-SIMS (Time-Of-Flight Secondary Ion MassSpectroscopy). Further, a surface compound can be identified by ameasurement of XRD (X-Ray Diffraction) or a measurement of the TOF-SIMS.

(1-2) Construction of Secondary Battery

Subsequently, an example of a construction of a secondary batteryaccording to the first embodiment of the invention will be describedwith reference to FIGS. 1 and 2.

FIG. 1 is a perspective view showing the example of the construction ofthe secondary battery according to the first embodiment. The secondarybattery has a construction in which a battery element 10 to which acathode lead 11 and an anode lead 12 have been attached is enclosed in afilm-shaped sheathing member 1.

The cathode lead 11 and the anode lead 12 are in, for example, astrip-shape and are led out from the inside of the sheathing member 1toward the outside, for example, in the same direction. The cathode lead11 is made of a metal material such as aluminum or the like. The anodelead 12 is made of a metal material such as nickel Ni or the like.

The sheathing member 1 has a structure in which, for example, aninsulating layer, a metal layer, and an outermost layer are laminated inthis order and adhered by laminate work or the like. The sheathingmember 1 is constructed in such a manner that, for example, the side ofthe insulating layer is set to the inside and outer edge portions aremutually adhered by melt-bonding or an adhesive agent.

The insulating layer is made of a polyolefin resin such as polyethylene,polypropylene, denatured polyethylene, denatured polypropylene, theircopolymer, or the like. This is because a moisture permeability can bereduced and excellent air-tightness is obtained. The metal layer is madeof foil-shaped or plate-shaped aluminum, stainless steel, nickel, iron,or the like. The outermost layer can be made of, for example, a resinsimilar to that of the insulating layer or may be made of nylon or thelike. This is because an intensity against a tear, piercing, or the likecan be enhanced. The sheathing member 1 may have a layer other than theinsulating layer, the metal layer, and the outermost layer.

An adhesive film 2 for improving adhesion between the cathode lead 11and the inside of the sheathing member 1 and adhesion between the anodelead 12 and the inside of the sheathing member 1 and preventing invasionof the outside atmosphere has been inserted between the sheathing member1 and each of the cathode lead 11 and the anode lead 12. The adhesivefilm 2 is made of a material having adhesion against each of the cathodelead 11 and the anode lead 12. For example, if the cathode lead 11 andthe anode lead 12 are made of the foregoing metal material, it ispreferable that they are made of the polyolefin resin such aspolyethylene, polypropylene, denatured polyethylene, denaturedpolypropylene, or the like.

FIG. 2 is a cross sectional view taken along the line II-II in thebattery element 10 shown in FIG. 1. The battery element 10 is formed bylaminating a cathode 13 and an anode 14 through a separator 15 and anelectrolyte 16 and winding them. An outermost peripheral portion of thebattery element 10 is protected by a protecting tape 17.

The cathode 13 has, for example, a cathode collector 13A and cathodeactive material layers 13B provided on both surfaces of the cathodecollector 13A. The cathode collector 13A is made of, for example, ametal foil such as an aluminum foil or the like.

The cathode active material layer 13B contains: the cathode activematerial according to the first embodiment; and a conductive materialsuch as a carbon material or the like and a binder such aspolyvinylidene fluoride, polytetrafluoro ethylene, or the like asnecessary.

In addition to the cathode active material according to the firstembodiment, the cathode active material layer 13B may also containanother cathode active material. As another cathode active material, forexample, a lithium nickel composite oxide containing lithium and nickel,a lithium manganese composite oxide having a spinel structure containinglithium and manganese, or a phosphate compound containing lithium andiron can be mentioned.

In a surface analysis by the Time-Of-Flight Secondary Ion MassSpectroscopy (TOF-SIMS), the cathode 13 has a peak of a fragment of atleast one or more secondary ions selected from positive secondary ionsof Li₄PO₄, Li₂CoPO₄, Li₂CoPH₂O₄, Li₃CoPO₄, and Li₃CoPO₄H and negativesecondary ions of PO₂, LiP₂O₄, Co₂PO₄, CoP₂O₅, CoP₂O₅H, CoP₂O₆, andCoP₂O₆H.

In a manner similar to the cathode 13, the anode 14 has, for example, ananode collector 14A and anode active material layers 14B provided onboth surfaces of the anode collector 14A. The anode collector 14A ismade of, for example, a metal foil such as a copper foil or the like.

The anode active material layer 14B is formed so as to contain one, two,or more kinds of anode materials in/from which lithium can be doped anddedoped as an anode active material. The anode active material layer 14Bmay contain a conductive material and a binder as necessary.

As an anode material in/from which lithium can be doped and dedoped, forexample, a carbon material such as graphite, non-easy-graphitizablecarbon, easy-graphitizable carbon, or the like can be mentioned. In thecarbon material, any one kind of elements can be solely used, two ormore kinds of elements may be mixed and used, or two or more kinds ofelements having different mean diameters can be also mixed and used.

As an anode material in/from which lithium can be doped and dedoped, amaterial containing a metal element or a semimetal element which canform an alloy together with lithium as a component element can bementioned. Specifically speaking, a simple substance, an alloy, or acompound of the metal element which can form an alloy together withlithium, a simple substance, an alloy, or a compound of the semimetalelement which can form an alloy together with lithium, or a materialhaving phases of one, two, or more kinds of them in at least a part canbe mentioned.

As such a metal element or a semimetal element, for example, thefollowing elements can be mentioned: tin Sn; lead Pb; aluminum; indiumIn; silicon Si; zinc Zn; antimony Sb; bismuth Bi; cadmium Cd; magnesiumMg; boron B; gallium Ga; germanium Ge; arsenic As; silver Ag; zirconiumZr; yttrium Y; or hafnium Hf. Among them, a metal element or a semimetalelement of Group 14 in a long period type periodic table is preferable.Silicon Si or tin Sn is particularly preferable. This is because in thecase of silicon Si or tin Sn, an ability of doping and dedoping lithiumis high and a high energy density can be obtained.

As an alloy of silicon, for example, an alloy containing at least onekind selected from a group containing tin Sn, nickel Ni, copper Cu, ironFe, cobalt Co, manganese Mn, zinc Zn, indium In, silver Ag, titanium Ti,germanium Ge, bismuth Bi, antimony Sb, and chromium Cr can be mentionedas a second component element other than silicon. As an alloy of tin Sn,for example, an alloy containing at least one kind selected from a groupcontaining silicon Si, nickel Ni, copper Cu, iron Fe, cobalt Co,manganese Mn, zinc Zn, indium In, silver Ag, titanium Ti, germanium Ge,bismuth Bi, antimony Sb, and chromium Cr can be mentioned as a secondcomponent element other than tin Sn.

As a compound of silicon Si or a compound of tin Sn, for example, acompound containing oxygen O or carbon C can be mentioned. In additionto silicon Si or tin Sn, the foregoing second component element can bealso contained.

As a separator 15, any material can be used so long as it iselectrically stable, is chemically stable against the cathode activematerial, the anode active material, or a solvent, and does not haveelectric conductivity. For example, a high molecular nonwoven fabriccloth, a porous film, or a paper-like sheet made of fiber of glass orceramics can be used, or a sheet obtained by laminating a plurality ofthose materials can be also used. Particularly, it is preferable to usea porous polyolefin film. A material obtained by combining such a filmwith a heat-resistant material made of fiber of polyimide, glass, orceramics, or the like.

The electrolyte 16 contains an electrolytic solution and a holdingmember containing a high molecular compound which holds the electrolyticsolution and is in what is called a gel-state. The electrolytic solutioncontains an electrolytic salt and a solvent which dissolves theelectrolytic salt. As an electrolytic salt, for example, lithium saltsuch as LiClO₄, LiPF₆, LiBF₄, LiN(SO₂CF₃)₂, LiN(SO₂C₂F₅)₂, LiAsF₆, orthe like can be mentioned. Although any one kind of them can be used asan electrolytic salt, two or more kinds of them may be mixed and used.

As a solvent, for example, the following solvents can be mentioned: alactone system solvent such as γ-butyrolactone, γ-valerolactone,δ-valerolactone, ε-caprolactone, or the like; a carbonic ester systemsolvent such as ethylene carbonate, propylene carbonate, butylenecarbonate, vinylene carbonate, dimethyl carbonate, ethylmethylcarbonate, diethyl carbonate, or the like; an ether system solvent suchas 1,2-dimethoxy ethane, 1-ethoxy-2-methoxy ethane, 1,2-diethoxy ethane,tetrahydrofuran, 2-methyl tetrahydrofuran, or the like; a nitrile systemsolvent such as acetonitrile or the like; a sulforan system solvent; aphosphoric ester solvent of a phosphate class or the like; or anon-aqueous solvent of a pyrrolidone class or the like. Any one kind ofthose solvents can be solely used as a solvent or two or more kinds ofthem can be also mixed and used.

It is preferable that the solvent contains a compound of fluorinatedcyclic or chain-like carbonate in which a part or all of hydrogen isfluorinated. It is preferable to use difluoro ethylene carbonate as afluorinated compound. This is because even in the case where the anode14 containing a compound such as silicon Si, tin Sn, germanium Ge, orthe like is used as an anode active material, charge/discharge cyclecharacteristics can be improved and, particularly, difluoro ethylenecarbonate is excellent in cycle characteristics improving effect.

The high molecular compound may be a compound which absorbs the solventand becomes a gel state. For example, the following compounds can bementioned: a fluorine system high molecular compound such aspolyvinylidene fluoride, copolymer of vinylidene fluoride and hexafluoropropylene, or the like; an ether system high molecular compound such aspolyethylene oxide, bridge member containing polyethylene oxide, or thelike; a compound containing polyacrylo nitrile, polypropylene oxide, orpolymethyl methacrylate as a repetitive unit; and the like. As a highmolecular compound, any one kind of those compound can be solely used ortwo or more kinds of them may be mixed and used.

Particularly, the fluorine system high molecular compound is preferablefrom a viewpoint of the oxidation-reduction stability. Among them, thecopolymer containing vinylidene fluoride and hexafluoro propylene ascomponents is preferable. Further, the copolymer may contain thefollowing material as a component: monoester of unsaturated dibasic acidsuch as monomethyl maleic ester or the like; ethylene halide such asethylene chloride trifluoride or the like; cyclic carbonate of anunsaturated compound such as vinylene carbonate or the like; acrylvinylmonomer containing an epoxy radical; or the like. This is because theadvanced characteristics can be obtained.

In the secondary battery having the above construction, an open circuitvoltage per pair of cathode and anode in the full charging state is setto, for example, 4.2V or a value over 4.2V. When the open circuitvoltage is set to a value over 4.2V, preferably, the open circuitvoltage lies within a range from 4.25V or more to 4.6V or less, muchpreferably, a range from 4.35V or more to 4.6V or less. This is becauseif an upper limit charge voltage of the secondary battery is set to ahigh voltage, a using ratio of the cathode active material can beincreased, a larger amount of energy can be extracted, and if it is setto 4.6V or less, the oxidation of the separator 15 can be suppressed.

(1-3) Manufacturing Method of Secondary Battery

An example of a manufacturing method of the secondary battery accordingto the first embodiment will now be described.

First, for example, the cathode active material layer 13B is formed onthe cathode collector 13A, thereby manufacturing the cathode 13. Thecathode active material layer 13B is formed as follows. For example, acathode mixture is adjusted by mixing powder of the cathode activematerial, a conductive material, and a binder. Thereafter, the cathodemixture is dispersed into the solvent such as N-methyl-2-pyrrolidone orthe like, thereby forming a paste-like cathode mixture slurry. Thecathode collector 13A is coated with the cathode mixture slurry and thesolvent is dried. Thereafter, the collector is compression-molded. Forexample, in a manner similar to the cathode 13, the anode activematerial layer 14B is formed on the anode collector 14A, therebymanufacturing the anode 14. Subsequently, the cathode lead 11 isattached to the cathode collector 13A and the anode lead 12 is attachedto the anode collector 14A.

Subsequently, an electrolytic solution and a high molecular compound aremixed by using a mixing solvent. The upper surface of the cathode activematerial layer 13B and the upper surface of the anode active materiallayer 14B are coated with the mixed solution and the mixing solvent isvolatilized, thereby forming the electrolyte 16. Subsequently, thecathode 13, separator 15, anode 14, and separator 15 are sequentiallylaminated and wound. The protecting tape 17 is adhered to the outermostperipheral portion, thereby forming the battery element 10. After that,the battery element 10 is sandwiched between the sheathing members 1 andthe outer edge portions of the sheathing members 1 are thermallymelt-bonded. In this instance, the adhesive films 2 are inserted betweenthe cathode lead 11 and the sheathing member 1 and between the anodelead 12 and the sheathing member 1, respectively. Thus, the secondarybattery shown in FIG. 1 is obtained.

The invention is not limited to the structure in which after theelectrolyte 16 was formed on the cathode 13 and the anode 14, they arewound. It is also possible to use a structure in which after the cathode13 and the anode 14 were wound through the separator 15 and they aresandwiched between the sheathing members 1, an electrolytic compositioncontaining an electrolytic solution and a monomer of a high molecularcompound is injected, and the monomer is polymerized in the sheathingmember 1.

According to such a secondary battery, when it is charged, for example,lithium ions are dedoped from the cathode 13 and doped into the anode 14through the electrolyte 16. On the other hand, when the battery isdischarged, for example, lithium ions are dedoped from the anode 14 anddoped into the cathode 13 through the electrolyte 16.

As described above, according to the first embodiment, since the surfacelayer is provided in at least a part of the lithium composite oxideparticle serving as a center portion, when the secondary battery in thecharging state was preserved under a high-temperature condition, thegeneration of the gas due to the reaction of the cathode 31 and theelectrolytic solution can be suppressed. The increase in internalresistance due to the reaction of the cathode 31 and the electrolyticsolution can be also suppressed.

Even in the case where the open circuit voltage per pair of cathode andanode in the full charging state is set to, for example, a value over4.2V, that is, a value within a range from 4.25V or more to 4.6V or lessor a range from 4.35V or more to 4.6V or less, a using ratio of thecathode active material is increased, and an electric potential of thecathode 13 is raised, the gas generation due to the reaction of thecathode 13 and the electrolytic solution can be suppressed. That is, thelarger amount of energy can be extracted and the high-temperaturepreserving characteristics can be remarkably improved.

Even in the secondary battery in which graphite which has widely beenused in the related art is used as an anode active material, theincrease in thickness of battery in the high-temperature preservationcan be suppressed. Such a suppressing effect is further typicallyobtained in the secondary battery in which the anode 14 using a compoundsuch as silicon Si, tin Sn, germanium Ge, or the like is used as ananode active material is used and fluorinated cyclic or chain-like esteris used as an electrolyte 16.

(2) Second Embodiment

A second embodiment is described below. A secondary battery according tothe second embodiment uses an electrolytic solution in place of the gelelectrolyte 16 in the secondary battery of the first embodiment. In thiscase, the electrolytic solution is impregnated into the separator 15. Asan electrolytic solution, an electrolytic solution similar to that inthe foregoing first embodiment can be used.

The secondary battery having such a construction can be manufactured,for example, as follows. In a manner similar to the foregoing firstembodiment except that the creation of the gel electrolyte 16 isomitted, the battery element 10 is formed by winding the cathode 13 andthe anode 14, the battery element 10 is sandwiched between the sheathingmembers 1, thereafter, the electrolytic solution is injected, and thesheathing member 1 is sealed.

In the second embodiment, an effect similar to that in the foregoingfirst embodiment can be obtained.

(3) Third Embodiment

(3-1) Cathode Active Material

Since a cathode active material according to the third embodiment issimilar to that in the foregoing first embodiment, its description isomitted.

(3-2) Construction of Secondary Battery

A Construction of a secondary battery according to the third embodimentis described below with reference to FIGS. 3 and 4.

FIG. 3 is a cross sectional view showing an example of the constructionof the secondary battery according to the third embodiment. Thesecondary battery is what is called a cylindrical type. In an almosthollow cylindrical battery can 21, the secondary battery has a windedelectrode member 30 in which a belt-shaped cathode 31 and a belt-shapedanode 32 have been wound through a separator 33. An electrolyticsolution as a liquid electrolyte has been impregnated in the separator33. The battery can 21 is made of, for example, iron Fe plated withnickel Ni. One end portion of the battery can 21 is closed and the otherend portion is open. In the battery can 21, a pair of insulating plates22 and 23 are arranged perpendicularly to a winded peripheral surface soas to sandwich the winded electrode member 30.

A battery cap 24 and a relief valve mechanism 25 and athermally-sensitive resistive (PTC: Positive Temperature Coefficient)element 26 provided in the battery cap 24 are attached to the open edgeportion of the battery can 21 by being caulked through a gasket 27. Theinside of the battery can 21 has been sealed. The battery cap 24 is madeof, for example, a material similar to that of the battery can 21. Therelief valve mechanism 25 is electrically connected to the battery cap24 through the PTC element 26. When an inner pressure of the batteryrises to a predetermined value or more by an internal short-circuit,heating from the outside, or the like, a disk plate 25A is reversed,thereby disconnecting the electric connection between the battery cap 24and the winded electrode member 30. When a temperature rises, the PTCelement 26 limits a current by an increase in resistance value, therebypreventing an abnormal heat generation that is caused by the largecurrent. The gasket 27 is made of, for example, an insulating materialand its surface is coated with asphalt.

The winded electrode member 30 is wound around, for example, a centerpin 34 as a center. A cathode lead 35 made of aluminum Al or the like isconnected to the cathode 31 of the winded electrode member 30. An anodelead 36 made of nickel Ni or the like is connected to the anode 32. Thecathode lead 35 is welded to the relief valve mechanism 25, so that itis electrically connected to the battery cap 24. The anode lead 36 iswelded to the battery can 21 and is electrically connected thereto.

FIG. 4 is a cross sectional view enlargedly showing a part of the windedelectrode member 30 shown in FIG. 3. The winded electrode member 30 isobtained by laminating the cathode 31 and the anode 32 through theseparator 33 and winding them.

The cathode 31 has, for example, a cathode collector 31A and cathodeactive material layers 31B provided on both surfaces of the cathodecollector 31A. The anode 32 has, for example, an anode collector 32A andanode active material layers 32B provided on both surfaces of the anodecollector 32A. Constructions of the cathode collector 31A, cathodeactive material layers 31B, anode collector 32A, anode active materiallayers 32B, separator 33, and electrolytic solution are similar to thoseof the cathode collector 13A, cathode active material layers 13B, anodecollector 14A, anode active material layers 14B, separator 15, andelectrolytic solution in the foregoing first embodiment, respectively.

(3-3) Manufacturing Method of Secondary Battery

An example of a manufacturing method of the secondary battery accordingto the third embodiment is described below.

The cathode 31 is manufactured as follows. First, a cathode mixture isadjusted by mixing the cathode active material, a conductive material,and a binder. The cathode mixture is dispersed into the solvent such as1-methyl-2-pyrrolidone or the like, thereby forming a cathode mixtureslurry. Subsequently, the cathode collector 31A is coated with thecathode mixture slurry and the solvent is dried. Thereafter, thecollector is compression-molded by a roll pressing machine or the like,thereby forming the cathode active material layers 31B and obtaining thecathode 31.

The anode 32 is manufactured as follows. First, an anode mixture isadjusted by mixing the anode active material and a binder. The anodemixture is dispersed into the solvent such as 1-methyl-2-pyrrolidone orthe like, thereby forming an anode mixture slurry. Subsequently, theanode collector 32A is coated with the anode mixture slurry and thesolvent is dried. Thereafter, the collector is compression-molded by theroll pressing machine or the like, thereby forming the anode activematerial layers 32B and obtaining the anode 32.

Subsequently, the cathode lead 35 is attached to the cathode collector31A by welding or the like and the anode lead 36 is attached to theanode collector 32A by welding or the like. Thereafter, the cathode 31and the anode 32 are wound through the separator 33. A front edgeportion of the cathode lead 35 is welded to the relief valve mechanism25 and a front edge portion of the anode lead 36 is welded to thebattery can 21. The wound cathode 31 and anode 32 are sandwiched betweenthe pair of insulating plates 22 and 23 and enclosed in the battery can21. After the cathode 31 and the anode 32 were enclosed in the batterycan 21, an electrolyte is injected into the battery can 21 andimpregnated into the separator 33. Thereafter, the battery cap 24,relief valve mechanism 25, and PTC element 26 are fixed to the open edgeportion of the battery can 21 by being caulked through the gasket 27. Inthis manner, the secondary battery shown in FIG. 3 is manufactured.

In the third embodiment, effects similar to those in the foregoing firstembodiment can be obtained.

(4) Fourth Embodiment

A secondary battery according to the fourth embodiment has aconstruction similar to that in the foregoing first embodiment exceptfor cathode active material layers. The same or similar componentelements as those in the foregoing first embodiment are designated bythe same reference numerals and explanation is made below.

In the cathode active material layer 13B, at least one kind selectedfrom the group consisting of sulfur S and phosphorus P is contained in aportion near the particle surface of the lithium composite oxide and acontent of the kind in the portion is largest in the cathode activematerial layer 13B. At least one kind of sulfur S and phosphorus P iscontained as, for example, a compound in the cathode active materiallayer 13B. More specifically speaking, sulfur S is contained as, forexample, Li₂SO₄ in the cathode active material layer 13B and phosphorusP is contained as, for example, Li₃PO₄ or LiCoPO₄ in the cathode activematerial layer 13B. The lithium composite oxide is similar to that inthe foregoing first embodiment.

It is preferable that the content of the kind in the portion lies withina range from 0.1 at % or more to less than 5 at % as a ratio to cobaltCo of the cathode active material. This is because if it is set to avalue of 0.1 at % or more, the excellent preserving characteristics canbe obtained and if it is set to a value less than 5 at %, the increasein internal resistance and the decrease in capacitance can besuppressed.

It is preferable that a center particle diameter of the cathode activematerial lies within a range from 1 μm or more to less than 30 μm. Thisis because if it is set to a value of 1 μm or more, the reaction of thecathode and the electrolytic solution can be suppressed, the increase ingas generation amount can be suppressed, and if it is set to a valueless than 30 μm, the sufficient capacitance and the excellent loadcharacteristics can be obtained.

It is preferable that a specific surface area lies within a range from0.1 m²/g or more to less than 1 m²/g. This is because if it is set to avalue of 0.1 m²/g or more, the sufficient capacitance, the excellentload characteristics can be obtained, and if it is set to a value lessthan 1 m²/g, the reaction of the cathode and the electrolytic solutioncan be suppressed, and the increase in gas generation amount can besuppressed.

As a method of confirming that in the cathode active material layer 13B,a large amount of sulfur S or phosphorus P exist in the portion near theparticle surface of the lithium composite oxide, a method whereby thecathode 13 is embedded into a resin and its cross sectional surface isexposed and, thereafter, distribution in the cross sectional surface isconfirmed by the TOF-SIMS can be mentioned. It can be also confirmed byanalyzing the elements by the XPS while argon-sputtering the surface ofthe cathode.

In a surface analysis by a Time-Of-Flight Secondary Ion MassSpectroscopy (TOF-SIMS), the cathode active material layer 13B has apeak of a fragment of at least one or more secondary ions selected frompositive secondary ions of Li₄PO₄, Li₂CoPO₄, Li₂CoPH₂O₄, Li₃CoPO₄, andLi₃CoPO₄H and negative secondary ions of PO₂, LiP₂O₄, Co₂PO₄, CoP₂O₅,CoP₂O₅H, CoP₂O₆, and CoP₂O₆H.

The cathode active material layer 13B can be manufactured, for example,as follows. By mixing the cathode active material, binder, conductivematerial, and at lease one kind of a sulfur-contained compound and aphosphorus-contained compound, a cathode mixture is adjusted and,thereafter, kneaded in N-methylpyrrolidone as a dispersion medium,thereby obtaining a paste-like cathode mixture slurry. The cathodecollector 13A is coated with the cathode mixture slurry, the slurry isdried, and the collector is compression-molded. In this manner, thecathode active material layer 13B is manufactured. As a sulfur-containedcompound and a phosphorus-contained compound, for example, compoundssimilar to those in the foregoing first embodiment can be used.

In the fourth embodiment, effects similar to those in the foregoingfirst embodiment can be obtained.

EXAMPLES

Although the embodiments are specifically described hereinbelow withrespect to Examples, they are not limited only to those Examples.

In Examples 1 to 8 and Comparison 1, the cathode active materialcontaining sulfur S or phosphorus P in the particle surface of thelithium composite oxide is used, secondary batteries are manufacturedwhile changing the content of sulfur S or phosphorus P to cobalt Co inthe cathode active material, and the high-temperature preservingcharacteristics are evaluated.

Example 1

A cathode is manufactured as follows. First, LiCoO₂ of 5000 g in which acenter particle diameter is equal to 12 μm and a specific surface areais equal to 0.20 m²/g and (NH₄)₂SO₄ of 34 g are mixed, the water isadded, and they are kneaded into a slurry shape. After that, the slurryis dried in a drier of 130° C., thermally processed for 3 hours at 900°C. in the air flow, thereafter broken, and sieved into the particles of75 μm or less, thereby manufacturing a cathode active material. A centerparticle diameter of the cathode active material is equal to 12.5 μm anda specific surface area is equal to 0.23 m²/g.

The center particle diameter is what is called a D50 particle diameterand is equal to a middle diameter (particle diameter which coincideswith an intermediate value of 50% of particle diameter distribution) ofthe particle diameter measured by a laser diffracting method (JISZ8825-1). The specific surface area is measured by a BET (BrunauerEmmett-Teller) method (JIS Z8830). In Examples and Comparisons, whichwill be described hereinbelow, the center particle diameters and thespecific surface areas are also obtained in a manner similar to thatmentioned above.

Subsequently, with respect to the manufactured cathode active material,element distribution is surface-analyzed by the SEM-EDS. Thus, sulfur Selements are detected at a larger mole ratio to an amount of thematerials used for manufacturing the cathode active material and it hasbeen found that many sulfur S elements exist in the surface of thecathode active material.

Subsequently, the manufactured cathode active material of 96 weightparts, polyvinylidene fluoride of 3 weight parts as a binder, and ketjenblack of 1 weight part as a conductive material are kneaded inN-methylpyrrolidone as a dispersion medium, thereby obtaining a cathodemixture slurry. The cathode collector having a thickness of 30 μm andmade of aluminum is coated with the cathode mixture slurry, the slurryis dried, and the collector is compression-molded by the roll pressingmachine, thereby forming a cathode active material layer andmanufacturing the cathode.

The cathode manufactured as mentioned above and a lithium metal anodeare laminated through a separator as a film made of polypropylene andwound in the longitudinal direction. After that, a protecting tape isadhered to the outermost peripheral portion, thereby manufacturing thebattery element.

Finally, the manufactured battery element is sandwiched between thesheathing members each made of an aluminum laminate film obtained bysandwiching an aluminum foil by polyolefin. Outer peripheral edgeportions excluding one side are thermally melt-bonded into a sack shape.The battery element is enclosed in the sheathing member. Subsequently,the electrolytic solution is injected into the sheathing member from thenon-bonded portion and the non-bonded portion of the sheathing member isthermally melt-bonded and sealed. As an electrolytic solution, LiPF₆ asa lithium salt is dissolved at a ratio of 1.0 mol/dm³ into a solventobtained by mixing ethylene carbonate and dimethyl carbonate at a massratio of 1:1 and an obtained solution is used. In this manner, the flatsecondary battery of the 500 mAh class is manufactured.

Example 2

A cathode active material is manufactured as follows. LiCoO₂ of 5000 gin which a center particle diameter is equal to 12 μm and a specificsurface area is equal to 0.20 m²/g and (NH₄)₂SO₄ of 69 g are mixed, thewater is added, and they are kneaded into a slurry shape. After that,the slurry is dried in the drier of 130° C., thermally processed for 3hours at 900° C. in the air flow, thereafter broken, and sieved into theparticles of 75 μm or less, thereby manufacturing the cathode activematerial. A center particle diameter of the cathode active material isequal to 11.6 μm and a specific surface area is equal to 0.22 m²/g.Subsequently, with respect to the manufactured cathode active material,element distribution is surface-analyzed by the SEM-EDS. Thus, thesulfur S elements are detected at a larger mole ratio to the amount ofthe materials used for manufacturing the cathode active material and ithas been found that many sulfur S elements exist in the surface of thecathode active material.

A secondary battery is manufactured in substantially the same manner asthat of foregoing Example 1 except that the cathode active materialmanufactured as mentioned above is used.

Example 3

A cathode active material is manufactured as follows. LiCoO₂ of 5000 gin which a center particle diameter is equal to 12 μm and a specificsurface area is equal to 0.20 m²/g and (NH₄)₂HPO₄ of 23 g are mixed, thewater is added, and they are kneaded into a slurry shape. After that,the slurry is dried in the drier of 130° C., thermally processed for 3hours at 900° C. in the air flow, thereafter broken, and sieved into theparticles of 75 μm or less, thereby manufacturing the cathode activematerial. A center particle diameter of the cathode active material isequal to 11.8 μm and a specific surface area is equal to 0.23 m²/g.Subsequently, with respect to the manufactured cathode active material,element distribution is surface-analyzed by the SEM-EDS. Thus, thephosphorus P elements are detected at a larger mole ratio to the amountof the materials used for manufacturing the cathode active material andit has been found that many phosphorus P elements exist in the surfaceof the cathode active material.

A secondary battery is manufactured in substantially the same manner asthat of foregoing Example 1 except that the cathode active materialmanufactured as mentioned above is used.

Example 4

A cathode active material is manufactured as follows. LiCoO₂ of 5000 gin which a center particle diameter is equal to 12 μm and a specificsurface area is equal to 0.20 m²/g and (NH₄)₂HPO₄ of 47 g are mixed, thewater is added, and they are kneaded into a slurry shape. After that,the slurry is dried in the drier of 130° C., thermally processed for 3hours at 900° C. in the air flow, thereafter broken, and sieved into theparticles of 75 μm or less, thereby manufacturing the cathode activematerial. A center particle diameter of the cathode active material isequal to 11.8 μm and a specific surface area is equal to 0.23 m²/g.Subsequently, with respect to the manufactured cathode active material,element distribution is surface-analyzed by the SEM-EDS. Thus, thephosphorus P elements are detected at a larger mole ratio to the amountof the materials used for manufacturing the cathode active material andit has been found that many phosphorus P elements exist in the surfaceof the cathode active material.

Subsequently, the cathode active material is analyzed by the TOF-SIMS asfollows. The cathode active material powder is embedded with a resin andcross-sectional work-processed by an argon ion milling apparatus.Thereafter, the TOF-SIMS analysis is performed. In the TOF-SIMSanalysis, “TOF-SIMSV” made by ION-TOF Co., Ltd. is used. Measuringconditions are set as follows: primary ions are set to 197Au+; anaccelerating voltage of an ion gun is set to 25 keV; unbunching; anirradiation ion current is set to 0.5 pA (measured by a pulse beam); apulse frequency is set to 50 kHz; a mass range is set to 1 to 200 amu; ascanning range is set to 25×25 μm; and spatial resolution is set to 0.2um. An analysis result is shown in FIG. 5. In FIG. 5, a gray portionshows an area containing CoO₂ and a white portion shows an areacontaining PO₃. It will be understood from FIG. 5 that many phosphoruscompounds exist in the surface of LiCoO₂ particle.

Subsequently, a secondary battery is manufactured in substantially thesame manner as that of foregoing Example 1 except that the cathodeactive material manufactured as mentioned above is used.

Example 5

A cathode active material is manufactured as follows. LiCoO₂ of 5000 gin which a center particle diameter is equal to 12 μm and a specificsurface area is equal to 0.20 m²/g, H₃PO₃ of 10 g, and ethanol of 1000mL are mixed and kneaded into a slurry shape. After that, the slurry isdried in the drier of 130° C., broken, and sieved into the particles of75 μm or less, thereby manufacturing the cathode active material. Acenter particle diameter of the cathode active material is equal to 12.2μm and a specific surface area is equal to 0.21 m²/g. Subsequently, withrespect to the cathode active material, element distribution issurface-analyzed by the SEM-EDS. Thus, the phosphorus P elements aredetected at a larger mole ratio to the amount of the materials used formanufacturing the cathode active material and it has been found thatmany phosphorus P elements exist in the surface of the cathode activematerial.

Subsequently, a secondary battery is manufactured in substantially thesame manner as that of foregoing Example 1 except that the cathodeactive material manufactured as mentioned above is used.

Example 6

A cathode active material is manufactured as follows. LiCoO₂ of 5000 gin which a center particle diameter is equal to 12 μm and a specificsurface area is equal to 0.20 m²/g, H₃PO₃ of 20 g, and ethanol of 1000mL are mixed and kneaded into a slurry shape. After that, the slurry isdried in the drier of 130° C., broken, and sieved into the particles of75 μm or less, thereby manufacturing the cathode active material. Acenter particle diameter of the cathode active material is equal to 12.2μm and a specific surface area is equal to 0.21 m²/g. Subsequently, withrespect to the cathode active material, element distribution issurface-analyzed by the SEM-EDS. Thus, the phosphorus P elements aredetected at a larger mole ratio to the amount of the materials used formanufacturing the cathode active material and it has been found thatmany phosphorus P elements exist in the surface of the cathode activematerial.

Subsequently, a secondary battery is manufactured in substantially thesame manner as that of foregoing Example 1 except that the cathodeactive material manufactured as mentioned above is used.

Example 7

A cathode is manufactured as follows. LiCoO₂ of 95.8 weight parts inwhich a center particle diameter is equal to 12 μm and a specificsurface area is equal to 0.20 m²/g, polyvinylidene fluoride of 3 weightparts as a binder, ketjen black of 1 weight part as a conductivematerial, and H₃PO₃ of 0.2 weight part are kneaded in N-methylpyrrolidone as a dispersion medium, thereby obtaining a cathode mixtureslurry. The cathode collector having a thickness of 30 μm and made ofaluminum is coated with the cathode mixture slurry, the slurry is dried,and the collector is compression-molded by the roll pressing machine,thereby forming a cathode active material layer and manufacturing thecathode.

Subsequently, the cathode is analyzed by the TOF-SIMS. Analysis resultsare shown in FIGS. 6 and 7. As shown in FIGS. 6 and 7, the peaks of thefragments based on the positive secondary ions of Li₄PO₄, Li₂CoPO₄,Li₂CoPH₂O₄, Li₃CoPO₄, and Li₃CoPO₄H and the negative secondary ions ofPO₂, LiP₂O₄, Co₂PO₄, CoP₂O₅, CoP₂O₅H, CoP₂O₆, and CoP₂O₆H are observed.Those results indicate that the compounds such as Li₃PO₄, LiCoPO₄, andthe like have been produced in the particle surface of the cathodeactive material.

Subsequently, the cathode is analyzed by the TOF-SIMS as follows. Thecathode is embedded with the resin and cross-sectional work-processed bythe argon ion milling apparatus. Thereafter, the TOF-SIMS analysis isperformed. In the TOF-SIMS analysis, “TOF-SIMSV” made by ION-TOF Co.,Ltd. is used. Measuring conditions are set as follows: the primary ionsare set to 197Au+; the accelerating voltage of the ion gun is set to 25keV; the unbunching; the irradiation ion current is set to 0.5 pA(measured by the pulse beam); the pulse frequency is set to 50 kHz; themass range is set to 1 to 200 amu; the scanning range is set to 25×25μm; and the spatial resolution is set to 0.2 um. An analysis result isshown in FIG. 8. It will be understood from FIG. 8 that the phosphoruscompound such as PO₃ exists so as to cover the LiCoO₂ particle.

Subsequently, a secondary battery is manufactured in substantially thesame manner as that of foregoing Example 1 except that the cathodemanufactured as mentioned above is used.

Example 8

A cathode is manufactured as follows. LiCoO₂ of 95.8 weight parts inwhich a center particle diameter is equal to 12 μm and a specificsurface area is equal to 0.20 m²/g, polyvinylidene fluoride of 3 weightparts as a binder, ketjen black of 1 weight part as a conductivematerial, and H₃PO₃ of 0.4 weight part are kneaded in N-methylpyrrolidone as a dispersion medium, thereby obtaining a cathode mixtureslurry. The cathode collector having a thickness of 30 μm and made ofaluminum is coated with the cathode mixture slurry, the slurry is dried,and the collector is compression-molded by the roll pressing machine,thereby forming a cathode active material layer and manufacturing thecathode.

The cathode is analyzed by the TOF-SIMS, so that the same peaks of thesecondary ion fragments as those in Example 7 are observed.

Subsequently, a secondary battery is manufactured in substantially thesame manner as that of foregoing Example 1 except that the cathodemanufactured as mentioned above is used.

(Comparison 1)

A cathode is manufactured as follows. LiCoO₂ of 96 weight parts in whicha center particle diameter is equal to 12 μm and a specific surface areais equal to 0.20 m²/g, polyvinylidene fluoride of 3 weight parts as abinder, and ketjen black of 1 weight part as a conductive material arekneaded in N-methylpyrrolidone as a dispersion medium, thereby obtaininga cathode mixture slurry. The cathode collector having a thickness of 30μm and made of aluminum is coated with the cathode mixture slurry, theslurry is dried, and the collector is compression-molded by the rollpressing machine, thereby forming a cathode active material layer andmanufacturing the cathode.

Subsequently, the cathode is analyzed by the TOF-SIMS. Analysis resultsare shown in FIGS. 6 and 7. As shown in FIGS. 6 and 7, the peaks of thefragments based on the positive secondary ions of Li₄PO₄, Li₂CoPO₄,Li₂CoPH₂O₄, Li₃CoPO₄, and Li₃CoPO₄H and the negative secondary ions ofPO₂, LiP₂O₄, Co₂PO₄, CoP₂O₅, CoP₂O₅H, CoP₂O₆, and CoP₂O₆H are notobserved. Subsequently, the cathode is analyzed by the TOF-SIMS in amanner similar to foregoing Example 7. An analysis result is shown inFIG. 9. It will be understood from FIG. 9 that no phosphorus compoundexists in the plane.

Subsequently, a secondary battery is manufactured in substantially thesame manner as that of foregoing Example 1 except that the cathodemanufactured as mentioned above is used.

(Evaluation of High-temperature Preserving Characteristics)

Subsequently, with respect to the secondary batteries manufactured asmentioned above, constant current charging is executed at a constantcurrent of 0.2 C until a battery voltage reaches 4.4V. After that,constant voltage charging is executed at a constant voltage of 4.4Vuntil a current value reaches a value corresponding to 0.01 C.Subsequently, constant current discharging is executed at a constantcurrent density of 0.2 C. until the battery voltage reaches 2.5V. Ahigh-temperature preserving test is performed as follows in order toevaluate the high-temperature preserving characteristics by separatingan amount of gas generation which is caused by the cathode. Thesecondary battery which was charged to 4.4V again after the firstcharging/discharging is decomposed. Only the cathode is taken out,wound, and packed to an aluminum laminate of 4 cm×5 cm. The pack ispreserved in a thermostat of 85° C. for 12 hours. After that, anincrease amount of a pack thickness is obtained from the packthicknesses measured before and after the preservation. The thicknessincrease amount is used as a scale of the amount of gas generationcaused by the cathode.

Subsequently, thickness change ratios in Examples 1 to 8 and Comparison1 are obtained by the following equation by using the thickness increaseamount obtained as mentioned above. Results are shown in Table 1. Thethickness change ratios are obtained on the assumption that the packthickness change ratio in Comparison 1 is set to 100%.Thickness change ratio (%) of aluminum laminate pack=[(pack thicknessincrease amount (mm) after the preservation at 85° C. for 12 hours ofeach Example)/(pack thickness increase amount (mm) after thepreservation at 85° C. for 12 hours in Comparison 1)]×100

TABLE 1 ADDITION FIRST THICKNESS ELEMENT MOLE EFFICIENCY CHANGE RATIO(at %) (%) RATIO (%) EXAMPLE 1 S/Co:0.5 94.5 69 EXAMPLE 2 S/Co:1.0 94.570 EXAMPLE 3 P/Co:0.3 94.7 63 EXAMPLE 4 P/Co:0.7 94.3 54 EXAMPLE 5P/Co:0.2 94.3 13 EXAMPLE 6 P/Co:0.4 94.5 14 EXAMPLE 7 P/Co:0.2 94.2 16EXAMPLE 8 P/Co:0.4 94.3 15 COMPARISON 1 — 94.5 100

It will be understood from Table 1 that in Examples 1 to 8, the increasein thickness of the laminate pack that is caused by the high-temperaturepreservation can be suppressed as compared with Comparison 1. Therefore,it will be understood that in Examples 1 to 8, the gas generation thatis caused by the reaction of the cathode and the electrolytic solutionin the full charging state can be suppressed.

In each of Example 9 and Comparison 2, a secondary battery ismanufactured by using the cathode formed by using the cathode activematerial particle containing phosphorus P in the surface, the anodecontaining graphite, and the electrolytic solution obtained by mixingethylene carbonate EC and diethyl carbonate DEC at a mass ratio of 3:7.The preserving characteristics are evaluated while changing an upperlimit voltage of this secondary battery.

Example 9

In an embodiment, the cathode is manufactured as follows. LiCoO₂ of 5000g in which a center particle diameter is equal to 12 μm and a specificsurface area is equal to 0.20 m²/g, H₃PO₃ of 10 g, and ethanol of 1000mL are mixed and kneaded into a slurry shape. After that, the slurry isdried in the drier of 130° C., broken, and sieved into the particles of75 μm or less, thereby manufacturing the cathode active material. Acenter particle diameter of the cathode active material is equal to 12.2μm and a specific surface area is equal to 0.21 m²/g. Subsequently, withrespect to the cathode active material, element distribution issurface-analyzed by the SEM-EDS. Thus, the phosphorus P elements aredetected at a larger mole ratio to the amount of the materials used formanufacturing the cathode active material and it has been found thatmany phosphorus P elements exist in the surface of the cathode activematerial. With respect to the cathode active material, an analysis ismade by the TOF-SIMS. Thus, many elements of Li₃PO₄, LiCoPO₄, and thelike exist in the portion near the surface of the cathode activematerial particle as compared with those in other portions.

Subsequently, the cathode active material of 96 weight partsmanufactured as mentioned above, polyvinylidene fluoride of 3 weightparts as a binder, and ketjen black of 1 weight part as a conductivematerial are kneaded in N-methylpyrrolidone as a dispersion medium,thereby obtaining a cathode mixture slurry. The cathode collector havinga thickness of 30 μm and made of aluminum is coated with the cathodemixture slurry, the slurry is dried, and the collector iscompression-molded by the roll pressing machine, thereby forming acathode active material layer and manufacturing the cathode.

The graphite serving as an anode active material and polyvinylidenefluoride PVDF as a binder are mixed at a mass ratio of 90:10, therebyforming an anode mixture. The anode mixture is dispersed into1-methyl-2-pyrrolidone, thereby forming an anode mixture slurry.Subsequently, the anode collector is coated with the anode mixtureslurry, the solvent is dried, and thereafter, the collector iscompression-molded by the roll pressing machine, thereby forming ananode active material layer and manufacturing an anode.

The cathode and the anode manufactured as mentioned above are laminatedthrough the separator as a film made of polypropylene and wound in thelongitudinal direction. After that, the protecting tape is adhered tothe outermost peripheral portion, thereby manufacturing the batteryelement.

Finally, the manufactured battery element is sandwiched between thesheathing members each made of the aluminum laminate film obtained bysandwiching the aluminum foil by polyolefin. Outer peripheral edgeportions excluding one side are thermally melt-bonded into a sack shape.The battery element is enclosed in the sheathing member. Subsequently,the electrolytic solution is injected into the sheathing member from thenon-bonded portion and the non-bonded portion of the sheathing member isthermally melt-bonded and sealed. As an electrolytic solution, LiPF₆ asa lithium salt is dissolved at a ratio of 1.0 mol/dm³ into a solventobtained by mixing ethylene carbonate EC and diethyl carbonate DEC at amass ratio of 3:7 and an obtained solution is used. In this manner, theflat secondary battery of the 500 mAh class is manufactured.

(Comparison 2)

The cathode is manufactured as follows. LiCoO₂ of 96 weight parts inwhich a center particle diameter is equal to 12 μm and a specificsurface area is equal to 0.20 m²/g, polyvinylidene fluoride of 3 weightparts as a binder, and ketjen black of 1 weight part as a conductivematerial are kneaded in N-methylpyrrolidone as a dispersion medium,thereby obtaining a cathode mixture slurry. The cathode collector havinga thickness of 30 μm and made of aluminum is coated with the cathodemixture slurry, the slurry is dried, and the collector iscompression-molded by the roll pressing machine, thereby forming acathode active material layer and manufacturing the cathode.

The secondary battery is manufactured in substantially the same manneras that of foregoing Example 9 except that the cathode manufactured asmentioned above is used.

(Evaluation of High-temperature Preserving Characteristics)

With respect to the secondary batteries manufactured as mentioned above,the charging is executed while changing the upper limit voltage as willbe explained hereinbelow, and the high-temperature preservingcharacteristics are evaluated. First, with respect to the batteriesmanufactured as mentioned above, the constant current charging isexecuted at the constant current of 0.2 C until the battery voltagereaches the upper limit voltages of 4.2V, 4.3V, and 4.4V. After that,the constant voltage charging is executed at the constant voltage ofeach of the above voltages until the current value reaches a valuecorresponding to 0.05 C. Subsequently, the constant current dischargingis executed at the constant current density of 0.2 C until the batteryvoltage reaches 2.7V. The battery which was charged again to the upperlimit voltages of 4.2V, 4.3V, and 4.4V after the firstcharging/discharging is preserved in the thermostat of 85° C. for 12hours. After that, a thickness increase amount is obtained from thebattery thicknesses measured before and after the preservation. Animpedance increase amount is obtained from AC impedance values(resistance values at 0.1 Hz) measured before and after thepreservation.

Subsequently, by substituting the thickness increase amount andimpedance increase amount obtained as mentioned above into the followingequations, a thickness change ratio and an impedance change ratio areobtained. Results are shown in Table 2. The thickness change ratio isobtained on the assumption that the thickness change ratio in Comparison2 is set to 100%. The impedance change ratio is obtained on theassumption that the impedance change ratio in Comparison 2 is set to100%.Battery thickness change ratio (%)=[(battery thickness increase amount(mm) after the high-temperature preservation in Example 9)/(batterythickness increase amount (mm) after the high-temperature preservationin Comparison 2)]×100Impedance change ratio (%)=[(impedance increase amount (Ω) after thehigh-temperature preservation in Example 9)/(impedance increase amount(Ω) after the high-temperature preservation in Comparison 2)]×100

TABLE 2 ELECTROLYTIC CHARGE UPPER THICKNESS IMPEDANCE SOLUTION LIMITVOLTAGE CHANGE CHANGE CATHODE ANODE COMPOSITIONS (%) RATIO (%) RATIO (%)EXAMPLE 9 CATHODE 1 GRAPHITE EC:DEC 4.2 75 80 EXAMPLE 9 CATHODE 1GRAPHITE EC:DEC 4.3 53 41 EXAMPLE 9 CATHODE 1 GRAPHITE EC:DEC 4.4 42 36COMPARISON 2 CATHODE 2 GRAPHITE EC:DEC 4.2 100 100 COMPARISON 2 CATHODE2 GRAPHITE EC:DEC 4.3 100 100 COMPARISON 2 CATHODE 2 GRAPHITE EC:DEC 4.4100 100 CATHODE 1: CATHODE FORMED BY USING CATHODE ACTIVE MATERIALPARTICLE CONTAINING PHOSPHORUS IN THE SURFACE CATHODE 2: CATHODE FORMEDBY USING CATHODE ACTIVE MATERIAL PARTICLE CONTAINING NO PHOSPHORUS INTHE SURFACE

It will be understood from Table 2 that in Example 9, the thicknessincrease amount and the impedance increase amount of the secondarybattery after the high-temperature preservation have been suppressed ascompared with those in Comparison 2. It will be also understood that inExample 9, an expansion amount suppressing effect of the cathodeincreases with an increase in upper limit voltage upon charging and alarger effect is obtained in the secondary battery whose energy densityhas been improved by raising the upper limit voltage.

In Examples 10 to 13 and Comparisons 3 to 5, the high-temperaturepreserving characteristics are evaluated while changing theconstructions of the cathode, anode, and electrolytic solution.

Example 10

A secondary battery is manufactured in substantially the same manner asthat of foregoing Example 9.

Example 11

The anode is manufactured as follows by a coating method by usingsilicon as an anode active material. Metal silicon (purity: 99%) brokenin a chip shape is pulverized into powder by a jet mill until its meandiameter reaches 1 μm. This silicon powder is dispersed into polyamideacid (polyimide precursor) of 3 weight %/NMP solution, thereby obtaininga slurry. An electrolytic copper foil as an anode collector is coatedwith the slurry. The slurry is dried and the collector iscompression-molded by the roll pressing machine. After that, a heatprocess is executed in a vacuum at 400° C. for 3 hours, therebymanufacturing the anode. In this state, initial slurry compositions areadjusted so that a weight ratio of silicon and polyimide is equal to90:10 based on the weight measurement.

A secondary battery is manufactured in substantially the same manner asthat of foregoing Example 9 except that the anode manufactured asmentioned above is used.

Example 12

The electrolytic solution is manufactured as follows. As an electrolyte,LiPF₆ as a lithium salt is dissolved at a ratio of 1.0 mol/dm³ into asolvent obtained by mixing difluoro ethylene carbonate DFEC, ethylenecarbonate EC, and diethyl carbonate DEC at a mass ratio of 5:25:70,thereby forming the electrolytic solution.

A secondary battery is manufactured in substantially the same manner asthat of foregoing Example 11 except that the electrolytic solutionmanufactured as mentioned above is used.

Example 13

The anode is manufactured as follows by an evaporation depositing methodby using silicon as an anode active material. While metal silicon(purity: 99%) similar to that used in Example 11 is used as a rawmaterial and an oxygen gas diluted by argon is introduced into achamber, a partially-oxidized amorphous silicon layer having a thicknessof 4 μm is formed onto the electrolytic copper foil having the coarsesurface by an electron beam evaporation depositing method, therebymanufacturing an anode.

A secondary battery is manufactured in substantially the same manner asthat of foregoing Example 12 except that the anode manufactured asmentioned above is used.

(Comparison 3)

A secondary battery is manufactured in substantially the same manner asthat of foregoing Comparison 2.

(Comparison 4)

The anode is manufactured in a manner similar to Example 11. A secondarybattery is manufactured in substantially the same manner as that offoregoing Comparison 3 except that the anode manufactured as mentionedabove is used.

(Comparison 5)

An electrolytic solution is manufactured in a manner similar to Example12. A secondary battery is manufactured in substantially the same manneras that of foregoing Comparison 4 except that the electrolytic solutionmanufactured as mentioned above is used.

(Evaluation of High-temperature Preserving Characteristics)

With respect to the batteries manufactured as mentioned above, theconstant current charging is executed at the constant current of 0.2 Cuntil the battery voltage reaches the upper limit voltage of 4.2V. Afterthat, the constant voltage charging is executed at the constant voltageof this voltage until the current value reaches a value corresponding to0.05 C. Subsequently, the constant current discharging is executed atthe constant current density of 0.2 C until the battery voltage reaches2.7V. The battery which was charged again to the upper limit voltageafter the first charging/discharging is preserved in the thermostat of85° C. for 12 hours. After that, a thickness increase amount is obtainedfrom the battery thicknesses measured before and after the preservation.Subsequently, by substituting the thickness increase amount obtained asmentioned above into the following equation, a thickness change ratio isobtained. A result is shown in Table 3. The thickness change ratio isobtained on the assumption that the battery thickness change ratio inComparison 2 is set to 100%.Battery thickness change ratio (%)=[(battery thickness increase amount(mm) after the high-temperature preservation in each of Examples 10 to13 and Comparisons 4 and 5)/(battery thickness increase amount (mm)after the high-temperature preservation in Comparison 3)]×100

(Evaluation of Cycle Characteristics)

With respect to the secondary batteries manufactured as mentioned above,the charging/discharging similar to those in the foregoing evaluation ofthe high-temperature preserving characteristics are repeated by 100cycles, thereby obtaining a discharge capacitance at the first cycle anda discharge capacitance at the 100th cycle. By substituting thedischarge capacitance values into the following equation, a capacitancemaintaining ratio after 100 cycles is obtained. A result is shown inTable 3.Capacitance maintaining ratio after 100 cycles (%)=[(dischargecapacitance (mAh) at the 100th cycle)/(discharge capacitance (mAh) atthe 1st cycle)]×100

TABLE 3 ELECTROLYTIC THICKNESS CAPACITANCE SOLUTION CHANGE MAINTAININGCATHODE ANODE COMPOSITIONS RATIO (%) RATIO (%) EXAMPLE 10 CATHODE 1GRAPHITE EC:DEC 75 95 EXAMPLE 11 CATHODE 1 Si COATING EC:DEC 40 41EXAMPLE 12 CATHODE 1 Si COATING DFEC:EC:DEC 111 91 EXAMPLE 13 CATHODE 1Si EVAPORATION-DEPOSITED DFEC:EC:DEC 119 91 COMPARISON 3 CATHODE 2GRAPHITE EC:DEC 100 94 COMPARISON 4 CATHODE 2 Si COATING EC:DEC 208 40COMPARISON 5 CATHODE 2 Si COATING DFEC:EC:DEC 803 90 CATHODE 1: CATHODEFORMED BY USING CATHODE ACTIVE MATERIAL PARTICLE CONTAINING PHOSPHORUSIN THE SURFACE CATHODE 2: CATHODE FORMED BY USING CATHODE ACTIVEMATERIAL PARTICLE CONTAINING NO PHOSPHORUS IN THE SURFACE

The following points can be understood from Table 3. When comparing theevaluation results of Comparisons 3 and 4, in the case where the anodein which silicon Si is used as an anode active material is used in placeof the anode in which graphite is used as an anode active material, itwill be understood that the thickness change ratio of thehigh-temperature preservation is increased by about two times and thecycle characteristics deteriorate fairly. When comparing the evaluationresults of Comparisons 4 and 5, in the secondary battery using the anodecontaining silicon Si as an anode active material, it will be understoodthat although the cycle characteristics are remarkably improved in thecase of using the electrolytic solution containing the fluorine systemsolvent, the high-temperature preserving characteristics deterioratesfairly.

When comparing the evaluation results of Examples 12 and 13, in thesecondary battery using the cathode formed by using the cathode activematerial particle containing phosphorus P in the surface, in the casewhere the anode containing silicon Si as an anode active material isused and the electrolytic solution containing a fluorine system solventis used, it will be understood that the high-temperature preservingcharacteristics and the cycle characteristics which are almost equal tothose in the secondary battery using the anode containing graphite as ananode active material can be also obtained.

When comparing the evaluation results of Example 10 and Comparison 3, inthe secondary battery using the anode containing graphite as an anodeactive material, it will be understood that by using the cathode formedby using the cathode active material particle containing phosphorus P inthe surface, the thickness change ratio of the high-temperaturepreservation can be suppressed into about 3/4. On the other hand, whencomparing the evaluation results of Example 12 and Comparison 5, in thesecondary battery using anode containing silicon as an anode activematerial, it will be understood that by using the cathode formed byusing the cathode active material particle containing phosphorus P inthe surface, the thickness change ratio of the high-temperaturepreservation can be suppressed into about 1/8. That is, it will beunderstood that the cathode formed by using the cathode active materialparticle containing phosphorus P in the surface provides the effect evenin the secondary battery using the anode containing graphite as an anodeactive material, the further large effect is obtained in the secondarybattery using silicon Si as an anode active material.

Although the embodiments and Examples of the invention have specificallybeen described above, the invention is not limited to the foregoingembodiments and Examples but various modifications based on thetechnical idea of the invention are possible.

For example, the numerical values mentioned in the foregoing embodimentsand Examples are nothing but examples and other numerical valuesdifferent from them can be also used as necessary.

Although the embodiments and Examples have been described above withrespect to the cases where the invention is applied to the flat typesecondary battery and the cylindrical secondary battery, the inventioncan be also applied to other secondary batteries of a rectangular type,a button type, a thin type, a large type, and a laminated type. Theinvention is not limited to the secondary battery but can be alsoapplied to a primary battery.

Although the foregoing embodiments and Examples have been described withrespect to a secondary battery using the simple substance of graphite orsilicon Si as an anode active material, the cathode active material orthe cathode can be also similarly applied to the secondary battery inwhich an alloy containing silicon Si, a mixture of silicon Si and carbonC, or a simple substance or a compound containing an element such as tinSn, germanium Ge, or the like is used as an anode active material.

In the above embodiments, for example, the lithium composite oxidehaving the stratified structure shown in Formula 2, the lithiumcomposite phosphate having the structure of the phosphate system shownin Formula 3, or the like can be also used as a lithium composite oxide.Li_(p)Ni_((1-q-r))Mn_(q)M1_(r)O_((2-y))X_(z)  (Formula 2)

in Formula 2, M1 denotes at least one kind of elements selected fromGroups 2 to 15 excluding Ni and Mn; X denotes at least one kind selectedfrom elements in Groups 16 and 17 excluding oxygen O; p indicates avalue within a range of 0≤p≤1.5; q indicates a value within a range of0≤q≤1.0; r indicates a value within a range of 0≤r≤1.0; y indicates avalue within a range of −0.10≤y≤0.20; and z indicates a value within arange of 0≤z≤0.2, andLi_(a)M2_(b)PO₄  (Formula 3)

in Formula 3, M2 denotes at least one kind of elements selected fromGroups 2 to 15; a indicates a value within a range of 0≤a≤2.0; and bindicates a value within a range of 0.5≤b≤2.0.

It should be understood that various changes and modifications to thepresently preferred embodiments described herein will be apparent tothose skilled in the art. Such changes and modifications can be madewithout departing from the spirit and scope of the present subjectmatter and without diminishing its intended advantages. It is thereforeintended that such changes and modifications be covered by the appendedclaims.

The invention is claimed as follows:
 1. A secondary battery comprising acathode, an anode, and an electrolyte, wherein the cathode comprisesparticles of a cathode active material, each particle comprising a corecomprising (i) a lithium composite oxide comprising lithium (Li) andcobalt (Co) and (ii) a first content of an element selected from thegroup consisting of sulfur (S), phosphorus (P), and a combinationthereof, and a surface layer on a surface of the core, the surface layercomprising the element in a second content more than the first contentof the element in the core, an atomic % ratio of the second content ofthe element to a content of cobalt (Co) in the cathode active materialranging from 0.1 to 5, the surface layer comprising the element in acompound selected from the group consisting of Li₂SO₄, LiCoPO₄, andcombinations thereof; the anode comprises an anode active materialincluding Si; and the electrolyte comprises a compound of fluorinatedcyclic or chain-like carbonate in which a part or all of hydrogen isfluorinated, and includes ethylene carbonate EC, diethyl carbonate DECand DFEC, the anode comprises an anode collector covered with the metalor semimetal material.
 2. The secondary battery according to claim 1,wherein the lithium composite oxide has an average composition expressedby Formula 1,Li_(x)Co_(1-y)M_(y)O_(b-a)X_(a)  (Formula 1) wherein M denotes anelement selected from the group consisting of boron (B), magnesium (Mg),aluminum (Al), silicon (Si), phosphorus (P), sulfur (S), titanium (Ti),chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni),copper (Cu), zinc (Zn), gallium (Ga), yttrium (Y), zirconium (Zr),molybdenum (Mo), silver (Ag), tungsten (W), indium (In), tin (Sn), lead(Pb), and antimony (Sb); X denotes a halogen element; and x, y, a, and brespectively indicate a value within a range of 0.2<x≤1.2, a valuewithin a range of 0≤y≤0.1, a value within a range of 1.8≤b≤2.2, and avalue within a range of 0≤a≤1.0.
 3. The secondary battery according toclaim 1, wherein a center particle diameter of the cathode activematerial ranges from 1 μm or more to less than 30 μm.
 4. The secondarybattery according to claim 1, wherein a specific surface area of thecathode active material ranges from 0.1 m²/g or more to less than 1m²/g.
 5. The secondary battery according to claim 1, wherein the metalmaterial comprises germanium (Ge).
 6. The secondary battery according toclaim 1, wherein the fluorinated compound comprises difluoro ethylenecarbonate.
 7. The secondary battery according to claim 1, wherein anopen circuit voltage per pair of the cathode and the anode in a fullcharging state ranges from 4.25V or more to 4.6V.
 8. The secondarybattery according to claim 1, wherein an open circuit voltage per pairof the cathode and the anode in a full charging state ranges from 4.35Vor more to 4.6V.
 9. The secondary battery according to claim 1, whereinthe particle of the lithium composite oxide, containing the phosphorus(P) in the particle surface, is obtained by preparing a solution bymixing a lithium composite oxide particle, H₃PO₃, and a solvent, anddrying the solution.
 10. A secondary battery comprising a cathode, ananode, and an electrolyte, wherein the cathode comprises particles of acathode active material, each particle comprising a core comprising alithium composite oxide comprising lithium (Li) and cobalt (Co) and asurface layer on a surface of the core, the surface layer comprising anelement selected from the group consisting of sulfur (S), phosphorus(P), and a combination thereof in a content that is the largest in thecathode active material, an atomic % ratio of the content of the elementto a content of cobalt (Co) in the cathode active material ranging from0.1 or more to less than 5, the surface layer comprising the element ina compound selected from the group consisting of Li₂SO₄, LiCoPO₄, andcombinations thereof; the anode comprises an anode active materialincluding Si; and the electrolyte comprises a compound of fluorinatedcyclic or chain-like carbonate in which a part or all of hydrogen isfluorinated, and includes ethylene carbonate EC, diethyl carbonate DECand DFEC, the anode comprises an anode collector covered with the metalmaterial.
 11. The secondary battery according to claim 10, wherein in asurface analysis by a Time-of-Flight Secondary Ion Mass Spectroscopy(ToF-SIMS), the cathode has peaks of fragments of secondary ionsselected from the group consisting of Li₄PO₄, Li₂CoPO₄, Li₂CoPH₂O₄,Li₃CoPO₄, and Li₃CoPO₄H and negative secondary ions of PO₂, LiP₂O₄,Co₂PO₄, CoP₂O₅, CoP₂O₅H, CoP₂O₆, CoP₂O₆H, and combinations thereof. 12.The secondary battery according to claim 1, wherein the lithiumcomposite oxide comprises LiCoO₂.
 13. The secondary battery according toclaim 1, wherein the metal material comprises silicon.
 14. The secondarybattery according to claim 1, wherein the compound comprises Li₂SO₄. 15.The secondary battery according to claim 1, wherein the compoundcomprises LiCoPO₄.
 16. The secondary battery according to claim 1,wherein the metal material further comprises Ti, and the anode collectorof the anode is covered with Si or Ti.
 17. The secondary batteryaccording to claim 1, wherein the anode comprises anode active materiallayers provided on both surfaces of the anode collector, the anodeactive material layers comprising an anode material comprising the metalor semimetal material.